The present invention relates to a method of heat-treating pipes and, more particularly, to a heat treating method for pipes capable of increasing the strength of the pipe against corrosion fatigue.
In modern nuclear plants, power stations and chemical plants, many straight, bent or other shaped pipes are connected, by welding or the like measures, so as to form continuous long pipe systems. However, especially in power stations, these pipes are used under strict conditions of high temperature and pressure, often causing the stress generated in the pipe to come up close to the yielding strength of the pipe material.
In general, these pipes are fabricated by plastic work and connected usually by welding. The residual tensile stresses caused by the plastic work and the welding are superimposed to a repetitive stress generated during the operation of the plant (e.g. repeated thermal stress), so as to cause a large compound stress. In addition, when a corrosive fluid is passed through the pipe system, it is necessary to take also the corrosion fatigue into consideration.
Hitherto, in order to diminish the residual stresses caused by the plastic work and the welding, various heat treating methods have been proposed and used in accordance with the kinds of materials of the pipes.
However, in these conventional heat treating method, tensile residual stress is left in the inner side of the pipe, due to the differential of cooling rate between the outer and the inner sides of the pipe during treating.
More specifically, in case of a pipe made of stainless steel, the heat treatment has been conducted in such a manner that the pipe is at first heated as a whole to a high temperature, and then dipped into a tank filled with cooling water. However, since the inner side of the pipe is not cooled until it is reached by the cooling water getting into the pipe through the end openings, the commencement of the cooling at the inside of the pipe is lagged behind that at the outer side of the pipe at which the cooling is started immediately after the dipping by the direct contact with the cooling water. In addition, the inner side of the pipe cannot contact sufficiently cold cooling water, while the temperature rise of the cooling water is not so high at the outer side of the pipe, so that the cooling rate is smaller at the inner side of the pipe than at the outer side, causing tensile residual stress in the inner side of the pipe. Furthermore, since there is a practical limit in the size of the pipe, only the essential parts of the pipe can be sufficiently cooled.
Thus, the conventional heat treatment cannot diminish the residual stress sufficiently well, so that the problem of rupture of pipe remains still unsolved, because of the poor strength against the corrosion fatigue, especially when a corrosive fluid is circulated though the pipe.